Voltage-level shifter

ABSTRACT

In a voltage-level shifter, an input line is configured to convey an input voltage to be shifted. A pair of transistors is coupled to and is configured to receive the input voltage from the input line. There is a first side and a second side, with each side comprising the following: a low-voltage transistor that is coupled to the pair of transistors, a medium-voltage transistor that is coupled to the low-voltage transistor, a high-voltage transistor that is coupled to the medium-voltage transistor, and an output line, which is coupled to the first and second sides, for providing an output voltage that is higher than the input voltage.

FIELD OF THE INVENTION

The present invention relates generally to integrated circuits, and specifically to a voltage-level shifter for an integrated circuit.

BACKGROUND OF THE INVENTION

Integrated circuits include many different components and are represented by many different designs. Examples of different designs are digital signal processors, central processing units, field-programmable gate arrays, memory, and so on. Non-volatile memory is one type of memory that preserves data with or without power. Manufacturers of non-volatile memory work continuously to improve the speed at which their memory operates and voltage shifters are one component in memory.

One problem with memory speed is found in the time it takes to shift lower input voltages to the higher voltages used by memory. Conventional voltage shifters shift relatively low voltage, for example a 1.8V logic signal, to a relatively high voltage, for example a 3.3V signal.

FIG. 1 is one example of a conventional voltage-level shifter 10. Shifter 10 receives a 1.8V signal at input 12 and “shifts” it to an output signal of 3.3V at output 14. Shifter 10 operates as follows.

Transistors 16 and 18 are thin-oxide, short-channel transistors that are inherently fast and small, but only tolerate voltage up to VDD from power supply 20. Transistors 16 and 18 are in an inverter configuration.

Transistors 22 and 24 are thick-oxide, long-channel transistors (relative to transistors 16 and 18) that can therefore tolerate higher voltage than transistors 16 and 18. Transistor 22 is connected to input 12 and receives the same input signal as transistors 18 and 16. Transistor 24, however, receives the inverted signal of input 12, because of the inverter configuration of transistors 16 and 18. Assuming input 12 is a high (VDD) voltage, then the gate of transistor 24 is deasserted (for example, a low voltage for N-channel transistors), while the gate of transistor 22 is asserted (for example, a high voltage for N-channel transistors).

Transistor 22 turns on, or begins conducting, because it is being asserted, while transistor 24 turns off because it's being deasserted. The effect of this is to turn on, or assert transistor 26 and turn off, or deassert transistor 28, which are both connected to power 30 at voltage level VCC, which is at 3.3V. Transistors 28 and 26 are thick-oxide, long, P-channel transistors (relative to transistors 16 and 18) that can therefore tolerate higher voltage than transistors 16 and 18. Because transistor 26 is on and conducting, while transistor 24 is off, output 14 is at VCC. Therefore the input voltage of 1.8V has been level-shifted to 3.3V. If input 12 goes to zero, then the opposite holds true, in that output 14 will go to zero as well.

One problem with voltage-level shifter 10 is that it is slow. In many electronic systems, memory being one example, rapidly functioning circuits are important to overall system performance.

Accordingly, what is needed is a faster voltage-level shifter. The present invention addresses such a need.

SUMMARY OF THE INVENTION

The present invention provides a voltage-level shifter comprising the following. In a voltage-level shifter, an input line is configured to convey an input voltage to be shifted. A pair of transistors is coupled to and is configured to receive the input voltage from the input line. There is a first side and a second side, with each side comprising the following: a low-voltage transistor that is coupled to the pair of transistors, a medium-voltage transistor that is coupled to the low-voltage transistor, a high-voltage transistor that is coupled to the medium-voltage transistor, and an output line, which is coupled to the first and second sides, for providing an output voltage that is higher than the input voltage.

According to the method and system disclosed herein, the present invention replaces the high-voltage, switching transistors with low-voltage transistors in series with medium-voltage transistors. The low-voltage transistors have very low “on” resistance and low capacitance, making them relatively fast, while the medium voltage transistors respond more quickly than the high-voltage transistors to an asserting signal. The overall effect of the replacement is to increase the conversion speed from input to output voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is one example of a conventional voltage-level shifter.

FIG. 2 is a schematic diagram illustrating a voltage-level shifter according to one embodiment of the invention.

FIG. 3 is a schematic diagram illustrating a voltage-level shifter according to one embodiment of the invention.

FIG. 4 is a schematic diagram illustrating one logic function implemented by a circuit of FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates generally to integrated circuits, and specifically to a voltage-level shifter for an integrated circuit. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiments and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features described herein.

FIG. 2 is a schematic diagram illustrating a voltage-level shifter 200 according to one embodiment of the invention. Shifter 200 receives an input voltage, for example 1.8V at input 202, and shifts the input voltage to an output voltage of 3.3V, for example, at output 204. Different input and output voltages may be used, with corresponding changes in transistor size when necessary. Shifter 200 operates as follows.

Transistors 206 and 208 are thin-oxide, short-channel transistors that are inherently fast and small, but only tolerate voltage up to VDD from power supply 210. In one embodiment, VDD may be 1.8V at the power supply node with transistors 206 and 208 having an oxide thickness of 32 Angstroms and a channel length of 0.18 μm. Transistor 208 is an N-channel transistor while transistor 206 is a P-channel transistor. Transistor pair 211 is in an inverter configuration.

Transistors 206 and 208 receive the input voltage from input 202. Because transistor pair 211 is configured as an inverter, transistor pair 211 outputs an inverted signal of input voltage. For example, if input voltage is high, transistor pair 211 outputs a low voltage, and vice versa.

Transistors 212, 214, 216, and 218 are also thin-oxide, short-channel transistors that are inherently fast and small, but only tolerate voltages up to VDD from power supply 210. In one embodiment, VDD may be 1.8V while transistors 212, 214, 216, and 218 have an oxide thickness of 32 Angstroms and a channel length of 0.78 μm. Transistors 214 and 218 are P-channel transistors while transistors 212 and 216 are N-channel transistors. Transistor pairs 220 and 222 are in inverter configurations.

Transistor pair 222 is connected to input 202 and receives the same input signal as transistor pair 211. Transistor pair 220, however, receives the inverted signal of input 202, because of the inverter configuration of transistor pair 211. Assuming input 202 is a high (VDD) voltage, then transistor pair 220 receives a logic low input (for example, a low voltage for N-channel transistors), while transistor pair 222 receives a logic high input (for example, a high voltage for N-channel transistors). Transistor pair 220 produces an assert signal while transistor pair 222 produces a deassert signal with a high (VDD) voltage input.

Circuits 224 and 226 have been described in FIG. 3 with greater detail. FIG. 3 is identical to FIG. 2 with the addition of a detailed embodiment of circuits 224 and 226. Circuits 224 and 226 are, in FIG. 3, identical to one another and produce the logic equivalent of FIG. 4. Circuits 224 and 226 receive input from transistor pairs 220 and 222, respectively, and output to transistors 228 and 230, respectively. For simplicity, the function of circuits 224 and 226 is next described with respect to the logic implemented.

FIG. 4 is a schematic diagram illustrating one logic function implemented by the circuits 224 and 226 of FIG. 3. Input circuit 300 may be either of transistor pairs 220 or 222 from FIG. 3 and is connected to NAND gate 310. The logical effect of inverters 320 and 325 is to cancel one another out, therefore NAND gate 310 transmits a high (VDD) voltage (or assert for N-channel transistors) to output circuit 340 when input circuit 300 is a low voltage (or deassert for N-channel transistors). Output circuit 340 may be either of transistors 228 or 230 from FIG. 2 or 3.

When input circuit 300 goes from high to low logic, there is a delay as NAND gate 310 receives the low input, implements it and outputs a high logic to output circuit 340. This delay is part of the normal operating characteristic of NAND gate 310. However, when input circuit 300 goes from low to high logic, there is an additional delay introduced by inverters 320 and 325, and capacitor 330. In order for NAND gate 310 to switch from a high logic output to a low logic output, both inputs to NAND gate 310 must be high, hence there is additional delay as inverters 325 and 320 process the signal and capacitor 330 discharges, and then NAND gate 310 receives both inputs as high logic. The significance of this additional delay, when switching from high logic input to low logic input, will be discussed below.

Returning to FIG. 3, circuits 224 and 226 are described in relation to the logic described in FIG. 4. The components of each of circuits 224 and 226 have been labeled and described together because in this embodiment their function is identical. Transistor block 232 includes transistors 234 and 236 and is configured as an inverter, for example inverter 320 of FIG. 4. Transistor block 238 includes transistors 240 and 242 and is configured as an inverter, for example inverter 325 of FIG. 4. Capacitor 244 is connected between transistor blocks 232 and 238 and functions as capacitor 330 from FIG. 4. Transistor block 250 includes transistors 252, 254, 256 and 258 and functions as NAND gate 310 from FIG. 4. The effect of circuits 224 and 226 is to receive a signal from transistor pairs 220 and 222 respectively, invert the signal and deliver it to transistors 228 and 230. The transistors in circuits 224 and 226 drive transistors 228 and 230.

Continuing with FIG. 2, transistors 228 and 230 have, for example, a medium oxide thickness (relative to transistors 206, 208, 212, 214, 216, and 218) of 90 Angstroms and a threshold voltage of approximately zero volts. Transistors 228 and 230 are N-channel transistors and protect transistor pairs 220 and 222 from excessive voltage, allowing them to be built from low-voltage transistors that are smaller, have less capacitance, and have a lower “on” resistance, and are therefore faster than those transistors in conventional systems. Transistors 228 and 212, and also transistors 230 and 216, are in series and may be considered a functional replacement for some of the transistors in conventional systems. The series combination of transistor 228, having a low threshold voltage than conventional systems, with transistor 212, which is a low-voltage transistor and highly conductive, is more conductive than the single high-voltage device in conventional systems.

Continuing with the example of a high (VDD) voltage signal at input 202, transistor pair 220 outputs a high (VDD) voltage signal (or assert signal in this embodiment) to circuit 224, while transistor pair 222 outputs a low-voltage signal (or deassert signal in this embodiment) to circuit 226. Circuit 224 produces a low-voltage (OV) signal to transistor 228 while circuit 226 produces a high-voltage (VDD) signal to transistor 230.

Continuing with the description of the circuit, transistors 260 and 262 are thick-oxide, long, P-channel transistors (relative to transistors 228 and 230) that can therefore tolerate higher voltage than transistors 228 and 230. Transistors 260 and 262 are cross-coupled to one another and connected to power supply 264, the voltage level to which the input voltage should be shifted, for example 3.3V.

Continuing with the example of a high (VDD) voltage signal at input 202, transistor 228 receives a low voltage, or deassert signal while transistor 230 receives a high voltage, or assert signal. Transistors 228 and 212 are being deasserted in this example while transistors 230 and 216 are being asserted. The gate of transistor 260 is pulled to ground and therefore asserted. Because transistors 228 and 212 are deasserted, voltage from power supply 264 is brought to output 204. Likewise, high voltage deasserts the gate of transistor 262.

Conversely, when input 202 is low, transistor 216 is deasserted and the output of transistor pair 222 is high. In this embodiment, in order to completely turn off transistor 230, the gate voltage of transistor 230 should reach zero volts with zero volts at input 202 and the line between transistors 216 and 230 should rise above zero volts, otherwise transistor 230 may leak current due to its low threshold voltage. As input 202 goes from high to low, transistors 216 and 218 switch states. Transistor 230 does not switch until some time has passed, in part because it is slower relative to transistors 216 and 218, and in part due to the previously discussed additional delay from circuit 226. With transistors 218 and 230 on, and transistor 216 off, the voltage brought up by transistor 218 assists in raising the gate voltage of transistor 260 and thereby speeding up the level conversion. After the delay for switching transistor 230 is over, transistor 230 shuts off, the gate of transistor 260 has been brought up somewhat by transistor 218 and will be brought up by transistor 262 until it shuts off.

Advantages of the invention include improving the speed of voltage-level conversion with thin-oxide, low voltage transistors. The invention applies generally to voltage-level shifters and specifically to shifting voltages from a 1.8V input signal to a 3.3V output signal in a non-volatile memory.

The present invention has been described in accordance with the embodiments shown, and one of ordinary skill in the art will readily recognize that there could be variations to the embodiments, and any variations would be within the spirit and scope of the present invention. The N and P-channel transistors are only exemplary, and one skilled in the art will recognize that each may be substituted for the other with subsequent design changes that are well known in the art. Also, the invention may be applied in any integrated circuit utilizing a level shifter. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims. 

1. A voltage-level shifter comprising: an input line for providing an input voltage to be shifted; a pair of transistors coupled to the input line for receiving the input voltage; a first side and a second side, each side comprising: a low-voltage transistor coupled to the pair of transistors; a medium-voltage transistor coupled to the low-voltage transistor; a high-voltage transistor coupled to the medium-voltage transistor; and an output line, coupled to each of the first and second sides, for providing an output voltage that is higher than the input voltage.
 2. The voltage-level shifter of claim 1, each of the low-voltage transistors comprising a thin-oxide, short-channel transistor.
 3. The voltage-level shifter of claim 1, the pair of transistors configured as an inverter.
 4. The voltage-level shifter of claim 1, wherein each of the medium voltage transistors protect each of the low-voltage transistors from excessive voltage.
 5. The voltage-level shifter of claim 1 further comprising for each of the first side and the second side: an inverter for inverting the signal from the low-voltage transistor to the medium-voltage transistor.
 6. The voltage-level shifter of claim 1, each of the medium-voltage transistors comprising medium oxide thickness transistors having a threshold voltage of approximately zero volts.
 7. A voltage-level shifter comprising: an input line configured to receive a signal; a first pair of transistors coupled to the input line for receiving the signal from the input line; a second pair of transistors coupled to the first pair of transistors for receiving a signal from the first pair of transistors; a third pair of transistors coupled to the input line for receiving the signal from the input line; a first transistor coupled to the second pair of transistors for receiving a signal from the second pair of transistors, the signal from the second pair of transistors deasserting the first transistor; a second transistor coupled to the third pair of transistors for receiving a signal from the third pair of transistors, the signal from the third pair of transistors asserting the second transistor; a fourth pair of transistors coupled to the first and second transistors for receiving a signal from the first and second transistors; and an output line coupled between the first transistor and the fourth pair of transistors for transmitting a voltage-level shifted signal when the first transistor is deasserted.
 8. The voltage-level shifter of claim 6, the first, second, and third pairs of transistors comprising thin-oxide, short-channel transistors.
 9. The voltage-level shifter of claim 7, the first, second, and third pairs of transistors configured as inverters.
 10. The voltage-level shifter of claim 8, the fourth pair of transistors cross-coupled to each other.
 11. The voltage-level shifter of claim 7, the first and second transistors having a higher voltage capacity than the second and third pairs of transistors, wherein the first and second transistors protect the second and third pairs of transistors from excessive voltage.
 12. The voltage-level shifter of claim 7, the fourth pair of transistors having a higher voltage capacity than the first and second transistors.
 13. The voltage-level shifter of claim 11, the fourth pair of transistors having a higher voltage capacity than the first and second transistors.
 14. The voltage-level shifter of claim 7 further comprising: a means for inverting the signal from the second pair of transistors to the first transistor; and a means for inverting the signal from the third pair of transistors to the second transistor.
 15. The voltage-level shifter of claim 7, the first and second transistors comprising medium-oxide transistors having a threshold voltage of approximately zero volts.
 16. A voltage-level shifter comprising: an input line for conveying an input voltage to be shifted; a pair of transistors coupled to the input line for receiving the input voltage from the input line; a first pair of low-voltage transistors coupled to the pair of transistors for receiving a signal from the pair of transistors; a second pair of low-voltage transistors coupled to the input line for receiving the input voltage; a first medium-voltage transistor coupled to the first pair of low-voltage transistors for receiving a signal from the first pair of low-voltage transistors; a second medium-voltage transistor coupled to the second pair of low-voltage transistors for receiving a signal from the second pair of low-voltage transistors; and a high-voltage transistor coupled to the first medium-voltage transistor for conveying an output voltage to an output line coupled between the high-voltage transistor and the first medium-voltage transistor while the input line receives the input voltage, wherein the output voltage is higher than the input voltage.
 17. The voltage-level shifter of claim 16, the first and second pair of low-voltage transistor comprising a thin-oxide, short-channel transistor.
 18. The voltage-level shifter of claim 16, the pair of transistors configured as an inverter.
 19. The voltage-level shifter of claim 16 further comprising: a means for inverting the signal from the first pair of low-voltage transistors to the first medium-voltage transistor, and from the second pair of low-voltage transistors to the second medium-voltage transistor.
 20. The voltage-level shifter of claim 16, the first and second medium-voltage transistors comprising medium oxide thickness transistors having a threshold voltage of approximately zero volts. 